Alexander Bain received a patent on an early facsimile machine a hundred and fifty years ago. The telegraph and facsimile came on the scene about the same time and had many things in common such as contact switching of metal patterns and interrupted current from the battery to convey information and both were received by marking on paper. In 1843, Bain obtained English patent number 9,745 for recording telegraph "facsimile unit" recorded on electrolytic paper. Bain used a pendulum for driving power and also to provide the clocking for timing operations. Casselli, in 1865, is believed to have used the first commercial facsimile which was sent over a long distance telegraph circuit. A satisfactory overall facsimile performance took many years with many people contributing to both the facsimile and other communication arts. Dr. Arthur Korn of Germany combined the tuning fork and synchronous motor and also started the optical scanning and photographic recording which are still used in today's fax systems. With the addition of vacuum tube amplifiers on telephone lines, telephone lines then became used for sending facsimile. AT&T provided a telephotography service which transmitted newspaper pictures by facsimile transmission. This has formed the basis for AT&T's telephoto equipment which was used by Associated Press, United Press International and Reuters. The Times Wide World Photo Service contacted Austin Cooly who worked on a facsimile for years to developed for them a semi-portable facsimile system. The resulting Cooly system demonstrated that good quality pictures could be sent even over regular unconditioned long distance telephone lines. A photographer taking a picture at one location could made a print of the picture for a facsimile operator. He then mounted it on a cylinder of a portable facsimile transmitter about the size of a small suitcase, a radical reduction of the size from a room of equipment used by Associated Press. A regular telephone call was placed to the Times picture darkroom from any telephone including the coin operated phone. A standard automobile battery served as a power source for the transmitter. The Times Facsimile Corporation (TFC) then began manufacturing and selling facsimile units to others. The military used it for weather map fax transmissions. The U.S. Army Air forces used the facsimile for weather map transmission during World War II and set up a world wide standard for weather facsimile. The International Telegraph and Telephone Consultive Committee (CCITT) initially adopted somewhat different standards, but abandoned them for use of those set at TFC. After the war, networks were set up for the broadcasting of facsimile weather charts over the telephone lines and over HF (High Frequency) radio. In the years that followed the war, various new facsimile equipment were specialized applications were designed and manufactured. The Electronics Industry Association (EIA) established a technical committee TR-29 on facsimile systems and equipment in the early 1960's. However, it was not until October 1966, before an EIA standard RS328 message facsimile equipment for operations switch voice facility using data communication equipment was published. This was the first U.S. "standard" on office fax. In 1967, with the AT&T Carterphone decision, permission was granted to allow direct connection to the PSTN (Public Switch Telephone Network) or the regular Bell telephone lines. The first group standard was Group 1 facsimile. This was generally better than earlier fax copies, facsimile units were still unreliable taking six minutes per page and were analog units. Users that accepted the quality of the Group 1 needed a faster system and the faster system became the Group 2 facsimile. This standard referred to as the three minute facsimile units already became the world wide acceptable system. Quality facsimile started with a digital facsimile. The earliest digital facsimile units used the adaptive run-length coding algorithm. Run-length coding removes redundancy from the page being sent and thus shortens the transmission time. In this system, a digital code word represents the number of successive white picture elements along a scanned line before the next black picture element (pixel or pel). The next code word represents the number of black pixels (picture elements) following a run of white picture elements. In 1980, the CCITT adopted a standard for digital facsimile which is referred to as Group 3. Although V.29 modem was intended to be used on 4 wire digital, facsimile units were used successfully using the V.29 modem with half-duplex on the PSTN and became an option in the Group 3 standard.
The standards for the Group 3 are Recommendations T.4 and T.30 in the CCITT Blue Book, including revisions completed in November 1988 and later. The main revisions of T.4 and T.30 are the addition of an optional error-free method of transmission and for two smaller-page versions of Group 3 fax.
Referring to FIG. 1, marked prior art, there is illustrated a conventional Group 3 facsimile unit. A scanner, such as a CCD (Charge Couple Device) scanner, reads the page being sent. The output is a series of picture elements (pels or pixels), and the amplitude of each pulse represents the brightness of the image on each element of the CCD chip or scanner. The scanner reads a very narrow line which may be in the order of 0.01 or 0.005 inch high, across the width of the page being sent. Scanning across the with of the page results in 1,728 pixels which results in a generation of 1,728 bits per line. A two-line memory stores the adjacent lines. The output from the scanner is in the analog signal and is converted by an A/D converter which produces one bit per picture element. The output from the A/D converter is digital and is passed through a Modified Huffman (MH) or Modified Read (MR) compression device that acts to compress the picture elements information to a small fraction of the number of bits. The Modified Huffman is a run-length coding and the modified read is a two-dimensional coding which further compresses the pixel or pel information. This coding is well known and is standard and is specified in CCITT Recommendation T.4 for Group 3. In a scanning line each sensor element has one bit of information to represent a black or white pixel or pel. Instead of sending a white line across a page as 1728 bits, the MH sends a 9-bit code word representing this. Since 1728, code words would have to be used to cover all run lengths the runs are grouped in multiples of 64 pels or pixels in a make-up code table. More detailed description of the code tables, etc., may be found various texts and in a book entitled "FAX: Digital Facsimile Technology & Applications" by Kenneth R. McConnell et al., published by Artech House, Inc., 685 Canton St., Norwood, Mass. 02062 and, in particular, this is shown in Section 2.5. This text is incorporated herein by reference. These codes are well known to those of ordinary skill in the art. The modified read is a relative addressing code uses vertical correlation with two adjacent lines on a page. For example, a black pel run on one line and a black pel line on the adjacent line uses one bit to provide that condition. The compressed digital output from the MH/MR compression, is stored in a buffer memory for use by the modem. The modem converts these signals into analog signals that can be sent over the regular telephone lines or PSTN. The modem is a modulation-demodulation device that accepts digital information and modulates it into the analog signal that is received by the telephone line. Of course, at the receiver, the analog signal is converted back to a run length coded binary data sent at the transmitter which is then expanded using a MH-MR expander and printed. The modem takes 4 bits at a time, for example, when operating at 9600 b/s, and represents them as one of 16 different states. CCITT recommendation V.29, V.27 ter specifies the modulation and demodulation schemes and the necessary portions of the CCITT modem built into the Group 3 facsimile unit. Recommendation V.27 ter is required for facsimile data at 4800 and 2400 b/s. Recommendation V.21 channel 2 (return channel) is required for the 300 b/s signaling. Recommendation V.29 is optional for facsimile data at 9600 and 7200 b/s for use on point-to-point four-wire leased type telephone channels and high-quality switched telephone channels. The Group 3 will usually try at first for 9600 b/s and if that is not successful will step down to 7200 b/s and attempt that. If necessary, this process can be repeated for the 4800 and 2400 b/s. When in the V.29 operating state at 9600 b/s the sampling of the incoming data facsimile signal is in 4-bit segments. As stated previously, there are 16 different analog signal states at the modem output which represents every possible arrangement of the 0s or 1s of the 4-bit segments at its input. FIG. 2 illustrates a V.29 signal space diagram at 9600 b/s. There are 16 different analog signal states at the modem output which represents every possible arrangement of the 0s or 1s of the four bit segments at its input. In the diagram Q represents the vertical coordinate and I represents the horizontal coordinate. Note that the phase is counter clock wise going from 0 to 360 about this coordinate axis and the amplitude is represented by the distance from the center of the diagram. From 0 to 90 degrees represents the first quadrant, 90 to 180 degrees represents the second quadrant, 180 to 270 degrees represents the third quadrant and 270 to 360 or 0 degrees represents the fourth quadrant. Each of the 16 dots represents a different signal state of the amplitude and phase. At 90 degrees, for example, the signal state 1010 has an amplitude of 3 and state 0010 has an amplitude of 2 as shown in FIG. 2. At 9600 b/s, the modem output changes 2,400 times per second for a baud rate of 2,400. The received PSTN analog signal from the network at the received modem compares the received signal dots to these standard dots in FIG. 2 and if the transmission is perfect these dots should coincide. Transmission impairments, such as phase jitter and noise can cause the received signals dots to move around form their assigned position and be taken to an adjacent signal state. This represents errors. A typical receiver an output of the modem is then converted to the expansion of MR/MR expansion to reproduce the binary midstream which represents the black and white pixels or pels from the CCD scanner for example. A terminal printer converts the bit stream into a printed copy of the original page. The thermal printer has closely space wires touching temperature responsive paper and heat is generated in a small spot on each wire when high current representing a black mark is passed to the wire.
In the middle of the 1980's, the growth of facsimile began to increase rapidly and the more the faxes became present the more the demand for storing the facsimile information became evident. The storage devices are sort of a facsimile mail boxes which receive data while party is away or receiving other facsimile requests and retrieved or discarded when the party returns or otherwise able to receive the facsimile. The party can receive multiple messages at the same time. The facsimile mail box systems do not actually print the expansion data and therefore have no need for the MH-MR expansion or to decode the scan lines and print the information.
It has also become desirable for the telephone central office to transmit the PSTN digitally using pulse code modulation where the amplitude of sampled voltage levels of the analog signal are represented by eight bit binary data.
It is desirable to determine the quality of the received signal at these facsimile mail boxes and to communicate when there is bad quality to the sender so that the rate may be slowed down or sent at another time or some other means may be done to improve the transmission quality. The transmission quality is normally determined as the percentage of bad scan lines. As discussed in the background facsimile machines use an encoding scheme (CCITT T.4) that sends a fixed number of picture elements per scan line. The copy quality of a Fax document is usually measured in the prior art as the percentage of bad scan lines. A line is considered bad if the decoded run-lengths in the scan line do not add up to 1728 picture elements or pixels. This implies that the scan lines must be at least partially decoded. This decoding in terms of decoding time and/or in terms of memory usage is expensive in terms of CPU (Central Processor Unit) resources. It takes a lot of memory space to store the decoding tables and cycle time to do the decoding.